Semiconductor nanocrystals are nanoparticles composed of an inorganic, crystalline semiconductive material and have unique photophysical, photochemical and nonlinear optical properties arising from quantum size effects, and have therefore attracted a great deal of attention for their potential applicability in a variety of contexts, e.g., as detectable labels in biological applications, and as useful materials in the areas of photocatalysis, charge transfer devices, and analytical chemistry. U.S. Pat. No. 6,821,337, the contents of which are incorporated herein by reference, discloses methods for making such nanocrystals. However, it has generally been found difficult to control the quality of such nanocrystals.
CdE, where E=S, Se, Te, are currently the most promising materials among the II-VI and III-V semiconductor nanocrystals suitable for applications based on their size-dependent emission. For example, CdSe nanocrystals emit from blue to red, depending on their size. Usually, photoluminescent properties are described by four parameters: emission wavelength, full width at half maximum (FWHM), intensity, and stability. The wavelength, which is directly related to the emission colour, is mainly determined by the nanocrystal size; furthermore, the FWHM depends strongly on size polydispersity. Because of the large surface-to-volume ratio, the photoluminescent efficiency, that is, intensity and stability, is primarily related to surface properties. Therefore, to target final performance, the focus should be on the synthesis of nanocrystals of an optimum size with a small size distribution as well as with few surface defects. The term, quantum dots, is used when the nanocrystals are nearly spherical in shape.
The first synthesis of CdE nanocrystals yielding a high quality product regarding the control of size, size distribution and crystallinity, was reported in 1993 (J. Am. Chem. Soc. 1993, 115, 8706-8715) using an organometallic approach. In this approach dimethylcadmium, Cd(CH3)2, is selected as the Cd source. Nucleation occurs at a high temperature (340-360° C.) in a coordinating solvent system consisting of a mixture of tri-n-octylphosphine (TOP) and tri-n-octylphosphine oxide (TOPO). Cd(CH3)2 is toxic, pyrophoric, expensive, unstable at room temperature in air, and explosive due to the release of gas during synthesis. This organometallic approach requires extremely restrictive synthetic equipment and conditions.
These problems can be partially overcome partially by using another synthetic route with an alternative Cd source developed in 2001 (J. Am. Chem. Soc. 2001, 123, 183-184), and described in the corresponding patent application entitled “Synthesis of Colloidal Nanocrystals” (PCT/US01/31239, or WO 02/29140 A1) filed on Oct. 04, 2001, the contents of which are herein incorporated by reference. In this approach, CdO and some Cd salts are used as the Cd precursors instead of Cd(CH3)2. It is claimed that the quality (i.e. the control of size(/shape) and distribution) of the CdSe dots prepared is comparable to those of the best reported in literature even without size selection. However, this method requires the use tri-n-octylphospine oxide and an acid.
Compared to Cd(CH3)2, CdO is less toxic, non-pyrophoric, less expensive, and safer to handle. The approaches using non-organometallic sources (such as CdO) are likely to become popular for the mainstream researchers as the more straightforward synthetic protocols that do not require the advanced degree of synthetic expertise; furthermore, these approaches provide the basis of an industrial-scale formulation for high-quality nanocrystals.
According to this approach an elemental chalcogen source in TOP (such as TOPSe) is added into a hot solution of CdO and tetradecylphosphonic acid (TDPA, excess) in TOPO at a temperature 250-360° C. It is claimed that a ligand (such as TDPA) with the molar ratio of the ligand to CdO larger than 1, had to be used. The reason given for this is the formation of a metal complex that is soluble in the coordinating solvent TOPO.
It is clearly indicated in the published patent application on page 3, lines 28-29, that “If CdO or CdCO3 is the precursor, amines and TOPO cannot be used as the ligands and can only be used as coordinating solvents.” Furthermore, “the presence of stearic acid was proven to be helpful for the formation of large-sized CdSe nanocrystals, which emit in the organe-red window.” (J. Am. Chem. Soc. 2002, 124, 2049-2055) Thus, the synthetic media in this approach involved five components, including two amines and one acid. (J. Am. Chem. Soc. 2002, 124, 2049-2055)
The JACS 2002 report referred to above discusses the evolution of the photoluminescent intensity of CdSe quantum dots produced via the non-organometallic approach using a complex of CdO and excess stearic acid. A corresponding patent application entitled “Colloidal Nanocrystals with high Photoluminescence Quantum Yields and Methods of Preparing the Same” (PCT/US02/24214 or WO 03/012006 A1) was filed on Jul. 30, 2002 and published on Feb. 13, 2003. The study shows that the photoluminescent intensity (quantum yield, QY) increases monotonically to a maximum value and then decreases during the growth of the dots from a single synthetic batch.
The maximum was termed a photoluminescent “bright point”. The photoluminescent intensity, represented by the quantum yield (QY), was based on the use of traditional dyes. However, it has been acknowledged that the demonstration of PL intensity by the QY is problematic, primarily because of the significant difference in QY values when different dyes are used. In addition, such a fundamental examination (JACS 2002) was based on materials synthesized from a five-component reaction medium involving the acid, two amines, TBP (tri-butylphosphine), and TOPO.
Photoluminescent properties are generally characterized by four parameters: emission wavelength (colour), full width at half maximum (FWHM) (colour purity), intensity (brightness), and stability. In order to improve the synthetic recipes and to develop optimal synthetic schemes for nanocrystals with desirable photoluminescent properties, it is necessary to note carefully what is known regarding the synthetic parameters affecting photoluminescent intensity. However, regarding the synthesis of “high quality” nanocrystals, present attention has been focused on controlling the size and size distribution. The term “high-quality” has not to date focused on the “long-term stability” and “sensitivity” of nanocrystals in various environments, such as polar and non-polar environments, both of which are important parameters relating to the quality of the nanocrystals.
After the first two reports (JACS 1993 and JACS 2001) on the synthesis of “high” quality” CdSe nanocrystals using Cd(Me)2 or CdO as the Cd precursors in, there is only one patent which was filed and opened but has not been granted, dealing with the synthesis of CdSe nanocrystals using CdO as the Cd precursor. The above PCT application claims that a ligand, in the form of an acid, is required.